1. Field of the Invention
This invention relates to a resist composition of the chemical amplification type for use in forming fine patterns upon manufacture of super LSIs.
2. Prior Art
With the advance of LSI technology, the memory capacity of IC chips reaches an order of mega bits and a rule of submicron is thus required for the fineness of wiring pattern. Accordingly, lithography light sources are shifting toward shorter wavelength from the ultraviolet band to the deep ultraviolet band which is more advantageous for fine patterning. Also, the current mainstream of etching step involved in the LSI manufacturing process is dry etching using RF plasma.
In these circumstances, the lithography technology uses resists which have to be sensitive and transmissive to the wavelength used and resistant against dry etching. More particularly, novolak type resins and other aromatic resins are useful resists for use in photolithography, especially g and i-ray lithography in view of light transmittance at the wavelength used and plasma etching resistance.
However, the light source intensity of deep ultraviolet light including mercury line spectrum on a shorter wavelength side than g and i-rays and excimer laser light such as KrF and ArF is extremely weaker than the light source intensity of g and i-rays. Utilization of such deep-UV light in photolithography has the problem that prior art resists conventionally used with g and i-rays provide unacceptable exposure sensitivity and low light transmittance to deep-UV light. There is a need for a new type of resist.
Resists of the chemical amplification type were developed as a substitute for the conventional resists. For example, Japanese Patent Application Kokai (JP-A) No. 45439/1984 discloses a resist composition comprising p-tert-butoxycarbonyloxy-.alpha.-methylstyrene which is one of polymers having acid unstable, recurring branched groups and a diaryl iodonium salt which is one of photo-polymerization initiators capable of generating acid upon exposure to radiation. Upon exposure of the resist to deep-UV light, the diaryl iodonium salt decomposes to generate an acid which in turn, causes cleavage of the p-tert-butoxy-carbonyl group of p-tert-butoxycarbonyloxy-.alpha.-methylstyrene into a polar group. Then a desired pattern is obtained by dissolving away the thus exposed areas or unexposed areas with base or non-polar solvent.
JP-A 115440/1987 discloses a resist composition comprising poly-4-tert-butoxy-.alpha.-methylstyrene and di(tert-butylphenyl) iodonium trifluoromethane sulfonate which are dissolved in diglyme and then exposed to deep-UV light. This resist achieves a high degree of resolution through the same reaction mechanism as the resist of JP-A 45439/1984.
Like the resist composition comprising poly-4-tert-butoxystyrene and an acid release agent disclosed in JP-A 115440/1987, JP-A 223858/1991 discloses a two-component system resist composition comprising a resin having a tert-butoxy group in its molecule and an acid release agent and JP-A 211258/1992 discloses a two-component system resist composition comprising a polyhydroxystyrene containing a methyl, isopropyl, tert butyl, tetrahydro-pyranyl or trimethylsilyl group in its molecule and an acid release agent.
The polymers used in these well-known resists as a major component are obtained by conventional radical or cationic polymerization of corresponding monomers. Since no special attention was paid to the molecular weight distribution of polymers, there were obtained polymers having a broad and uneven molecular weight distribution. Another base polymer of the chemical amplification type used in conventional resist compositions is a hydroxy-styrene resin in which some hydroxyl groups are replaced by tert-butoxycarbonyloxy groups (abbreviated as t-Boc groups). This resin is thermally unstable and the t-Boc groups are readily split off.
In these two-component system positive resist compositions comprising a hydroxystyrene resin in which hydroxyl groups are protected with protective groups and an acid release agent, many protective groups must be decomposed in exposed areas before the resist film can be dissolved in a developer. This undesirably leaves the increased possibility of changing film thickness and introducing stresses and bubbles in the film.
A number of chemical amplification type positive resist compositions have been proposed as mentioned above although they have some problems and are difficult to use in practice.